1. Field of the Invention
The present invention relates generally to the field of optical encoders, and, more particularly, to a low cost optical encoder of the reflective type capable of providing moderate resolution while requiring only a minimum number of wide tolerance parts.
2. Description of the Prior Art
Optical encoders are widely utilized in industry to measure linear or angular displacements of machine tools or other components. Basically, such systems operate by detecting and measuring the movement of markings on a moveable element as that element moves relative to a fixed reference. In conventional encoders, the moveable element comprises a code disc coupled to the machine component being monitored and having one or more code tracks thereon in the form of periodic patterns of alternating radiation transmissive and radiation opaque areas or increments. A source of illumination is usually placed on one side of the code disc and directed through the pattern on the disc. A stationary aperture or slit plate having a pattern similar to that of the code disc is then inserted between the code disc and a suitable detector located on the opposite side of the code disc from the radiation source. As the code disc rotates, the combination of disc and slit patterns will modulate the radiation incident upon the detectors and this modulation is detected and processed to form the coded word representing shaft position.
With this basic construction, encoders can read relatively fine code patterns and can deliver very accurate results (for example, code patterns of 1,000 cycles per inch or more are not uncommon). For encoders of this type to provide the required accuracy, however, they must be manufactured within very narrow tolerances. Specifically, each of the components in the encoder must be very accurately positioned and aligned relative to one another during manufacture. Furthermore, because encoders are frequently utilized in fairly severe environments, for example, in machines which vibrate substantially or undergo significant temperature variations, a substantial amount of care must be taken to ensure that the encoder remains accurately aligned at all times. Because of the precision required in such encoders, they are generally quite expensive and this limits the practicality of their use in industry.
In addition, for many applications, it is desirable to be able to obtain a resolution that is better than can be effectively obtained by merely reading the code patterns on the code plate. Specifically, the number of cycles (one cycle being an adjacent light transmissive and light opaque area of the code pattern) that can be placed on a code disc is limited by diffraction effects as well as by mechanical tolerances. To overcome this problem, a system has been previously developed by which a single cycle on a code disc can be electronically broken down into several parts. This concept, which is described in detail in, for example, U.S. Patent Nos. 3,265,902, 3,303,347 and 3,312,828, basically consists of providing two or more detectors positioned relative to one another and relative to the code disc such that the radiation striking them after passing through the code and slit plates will be 90.degree. out-of-phase with respect to a code track cycle. These two out-of-phase signals can be considered as sine and cosine signals, and, as such, can be electronically processed, as by a vector summing procedure, to provide several signals which effectively break a cycle into several smaller parts to provide greater resolution.